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    <meta charset="UTF-8">
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    <title>Document</title>
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<body>
    <script>
        // 将True变量的值赋值给flag变量 True变量不存在 直接报错
        // var flag = True;
        var flag = true;
        var flag2 = false;
        console.log(flag)
        console.log(typeof flag);//boolean

        var user;
        console.log(typeof user);//undefined

        var obj = null;
        console.log(typeof obj);//object 因为js中一切皆对象
    </script>
</body>
</html>